public Duct CreateDuctBetweenConnectors(UIDocument uiDocument)
{
    // prior to running this example
    // select some mechanical equipment with a supply air connector
    // and an elbow duct fitting with a connector in line with that connector
    Connector connector1 = null, connector2 = null;
    ConnectorSetIterator csi = null;
    ICollection<ElementId> selectedIds = uiDocument.Selection.GetElementIds();
    Document document = uiDocument.Document;
    // First find the selected equipment and get the correct connector
    foreach (ElementId id in selectedIds)
    {
        Element e = document.GetElement(id);
        if (e is FamilyInstance)
        {
            FamilyInstance fi = e as FamilyInstance;
            Family family = fi.Symbol.Family;
            if (family.FamilyCategory.Name == "Mechanical Equipment")
            {
                csi = fi.MEPModel.ConnectorManager.Connectors.ForwardIterator();
                while (csi.MoveNext())
                {
                    Connector conn = csi.Current as Connector;
                    if (conn.Direction == FlowDirectionType.Out && 
                        conn.DuctSystemType == DuctSystemType.SupplyAir)
                    {
                        connector1 = conn;
                        break;
                    }
                }
            }
        }
    }
    // next find the second selected item to connect to
    foreach (ElementId id in selectedIds)
    {
        Element e = document.GetElement(id);
        if (e is FamilyInstance)
        {
            FamilyInstance fi = e as FamilyInstance;
            Family family = fi.Symbol.Family;
            if (family.FamilyCategory.Name != "Mechanical Equipment")
            {
                csi = fi.MEPModel.ConnectorManager.Connectors.ForwardIterator();
                while (csi.MoveNext())
                {
                    if (null == connector2)
                    {
                        Connector conn = csi.Current as Connector;

                        // make sure to choose the connector in line with the first connector
                        if (Math.Abs(conn.Origin.Y - connector1.Origin.Y) < 0.001)
                        {
                            connector2 = conn;
                            break;
                        }
                    }
                }
            }
        }
    }

    Duct duct = null;
    if (null != connector1 && null != connector2)
    {
        // find a duct type
        FilteredElementCollector collector = new FilteredElementCollector(uiDocument.Document);
        collector.OfClass(typeof(DuctType));

        // Use Linq query to make sure it is one of the rectangular duct types
        var query = from element in collector
                    where element.Name.Contains("Mitered Elbows") == true
                    select element;

        // use extension methods to get first duct type
        DuctType ductType = collector.Cast<DuctType>().First<DuctType>();

        if (null != ductType)
        {
            duct = uiDocument.Document.Create.NewDuct(connector1, connector2, ductType);
        }
    }

    return duct;
}

Public Function CreateDuctBetweenConnectors(uiDocument As UIDocument) As Duct
    ' prior to running this example
    ' select some mechanical equipment with a supply air connector
    ' and an elbow duct fitting with a connector in line with that connector
    Dim connector1 As Connector = Nothing, connector2 As Connector = Nothing
    Dim csi As ConnectorSetIterator = Nothing
    Dim selectedIds As ICollection(Of ElementId) = uiDocument.Selection.GetElementIds()
    Dim document As Document = uiDocument.Document
    ' First find the selected equipment and get the correct connector
    For Each id As ElementId In selectedIds
        Dim e As Element = document.GetElement(id)
        If TypeOf e Is FamilyInstance Then
            Dim fi As FamilyInstance = TryCast(e, FamilyInstance)
            Dim family As Family = fi.Symbol.Family
            If family.FamilyCategory.Name = "Mechanical Equipment" Then
                csi = fi.MEPModel.ConnectorManager.Connectors.ForwardIterator()
                While csi.MoveNext()
                    Dim conn As Connector = TryCast(csi.Current, Connector)
                    If conn.Direction = FlowDirectionType.Out AndAlso conn.DuctSystemType = DuctSystemType.SupplyAir Then
                        connector1 = conn
                        Exit While
                    End If
                End While
            End If
        End If
    Next
    ' next find the second selected item to connect to
    For Each id As ElementId In selectedIds
        Dim e As Element = document.GetElement(id)
        If TypeOf e Is FamilyInstance Then
            Dim fi As FamilyInstance = TryCast(e, FamilyInstance)
            Dim family As Family = fi.Symbol.Family
            If family.FamilyCategory.Name <> "Mechanical Equipment" Then
                csi = fi.MEPModel.ConnectorManager.Connectors.ForwardIterator()
                While csi.MoveNext()
                    If connector2 Is Nothing Then
                        Dim conn As Connector = TryCast(csi.Current, Connector)

                        ' make sure to choose the connector in line with the first connector
                        If Math.Abs(conn.Origin.Y - connector1.Origin.Y) < 0.001 Then
                            connector2 = conn
                            Exit While
                        End If
                    End If
                End While
            End If
        End If
    Next

    Dim duct As Duct = Nothing
    If connector1 IsNot Nothing AndAlso connector2 IsNot Nothing Then
        ' find a duct type
        Dim collector As New FilteredElementCollector(uiDocument.Document)
        collector.OfClass(GetType(DuctType))

        ' Use Linq query to make sure it is one of the rectangular duct types
        Dim query As System.Collections.Generic.IEnumerable(Of Autodesk.Revit.DB.Element)
        query = From element In collector _
                Where element.Name.Contains("Mitered Elbows") = True _
                Select element

        ' use extension methods to get first duct type
        Dim ductType As DuctType = collector.Cast(Of DuctType)().First()

        If ductType IsNot Nothing Then
            duct = uiDocument.Document.Create.NewDuct(connector1, connector2, ductType)
        End If
    End If

    Return duct
End Function